1. Field of the Invention
This invention relates to a bearing device having a sleeve fitted to a shaft and supporting it for rotation relative to the shaft, and a deflecting-scanning apparatus using the same.
2. Related Background Art
Heretofore, a deflecting-scanning apparatus of this kind has been used in an image forming apparatus such as a laser beam printer or a laser facsimile apparatus, and to make this deflecting-scanning apparatus highly accurate, a highly accurately rotatable bearing device has been required, and a dynamic pressure fluid bearing rotatable in a non-contact manner is used in such a bearing device.
FIG. 1 of the accompanying drawings is a cross-sectional view of a deflecting-scanning apparatus using a bearing device by a dynamic pressure fluid bearing which is disclosed in Japanese Laid-Open Patent Application No. 8-5951. A fixed shaft 22 formed of a ceramic material is fixed to the housing 21 of a drive motor, and a rotatable sleeve 24 formed of a ceramic material is rotatably fitted to the fixed shaft 22. Also, a flange 25 formed of aluminum or brass is fixed to the outer periphery of the rotatable sleeve 24 as by shrinkage fitting, and a driving magnet 26 is adhesively secured to the outer periphery of this flange 25. Further, a stator 28 is disposed on a base plate 27 fixed onto the housing 21 so as to be opposed to the driving magnet 26, whereby a drive motor is constituted.
On the other hand, a second permanent magnet 30 is mounted on the upper end of the fixed shaft 22, and a first permanent magnet 29 is fixed to the rotatable sleeve 24 so that magnetic poles of different kinds may be vertically (axially of the fixed shaft) opposed to the second permanent magnet 30.
Thus, the rotatable sleeve 24 is floated up by a magnetic repulsive force and an air pool 31 is created between the fixed shaft 22 and the first permanent magnet 29. Also, a through-hole 23a for communicating the air pool 31 with the outside is provided in the first permanent magnet 29, and a plug 32 is removably mounted in this through-hole 23a. Further, a rotatable polygon mirror 34 is fixed onto the flange 25 by a leaf spring 33 fixed to the rotatable sleeve 24.
When with such a construction, the fixed shaft 22 and the rotatable sleeve 24 are fitted together with the plug 32 being removed, they can be easily fitted together with the air in the rotatable sleeve 24 flowing out of the through-hole 23a. Also, when the rotatable sleeve 24 is rotated, the rotatable sleeve 24 is supported in a radial direction by the air film between the rotatable sleeve 24 and the fixed shaft 22, and is supported in a thrust direction by the repulsive forces of the permanent magnets 29 and 30. At this time, the air in the air pool 31 enveloped by the plug 32 acts so as to attenuate the vertical movement of the rotatable sleeve 24, and holds the rotatable sleeve 24 in its stable floated-up position.
As described above, in this example of the prior art, the two permanent magnets for thrust floating-up are provided so as to be opposed to each other axially of the fixed shaft.
In such a construction, the rotatable sleeve 24 is very unstably vibrated vertically (axially) relative to a disturbance such as vibration to the apparatus. The vertical vibration of the rotatable sleeve is alleviated to some extent by the air pool 31, but it alone is insufficient.
Such vibration of the rotatable sleeve provides the vertical movement of the rotatable polygon mirror mounted thereon, and binders good deflection and scanning.
It is an object of the present invention to solve the above-noted problem and to provide a bearing device in which the disposition of permanent magnets for thrust floating-up is contrived to thereby prevent the vibration of a rotor portion (a rotatable sleeve or a rotary shaft), and a deflecting-scanning apparatus using the same.
To achieve the above object, a bearing device according to the present invention is structured to include a stator portion which includes one of a shaft and a sleeve, a rotor portion which includes the other of the shaft and the sleeve which are structured to be capable of relative rotation therebetween, a first permanent magnet mounted on the rotor portion, and a second permanent magnet mounted at a position opposed to the first permanent magnet, and wherein the rotor portion is floated up relative to the stator portion by a magnetic force working between the first permanent magnet and the second permanent magnet, and wherein the first permanent magnet is mounted on the upper end portion of the rotor portion, the second permanent magnet is mounted on the upper end portion of the stator portion, and the first permanent magnet and the second permanent magnet are provided in opposed relationship with each other in a circumferential direction perpendicular to an axial direction of the bearing device.
Also, a deflecting-scanning apparatus according to the present invention is structured to include a light source, a deflector for deflectively scanning a light beam from the light source, and a rotating device for rotatively driving the deflector. The bearing device of the rotating device is structured to include a stator portion which includes one of a shaft and a sleeve, a rotor portion which includes the other of the shaft and the sleeve which are structured to be capable of relative rotation therebetween, a first permanent magnet mounted on the rotor portion, and a second permanent magnet mounted at a position opposed to the first permanent magnet, and wherein the rotor portion is floated up relative to the stator portion by a magnetic force working between the first permanent magnet and the second permanent magnet, and wherein the first permanent magnet is mounted on the upper end portion of the rotor portion, the second permanent magnet is mounted on the upper end portion of the stator portion, and the first permanent magnet and the second permanent magnet are provided in opposed relationship with each other in a circumferential direction perpendicular to an axial direction of the bearing device.
Also, to achieve the above object, the bearing device according to the present invention is structured to include a stator portion which includes one of a shaft and a sleeve, a rotor portion which includes the other of the shaft and the sleeve which are structured to be capable of relative rotation therebetween, a first permanent magnet mounted on the rotor portion, and a second permanent magnet mounted at a position opposed to the first permanent magnet, and wherein the rotor portion is floated up relative to the stator portion by a magnetic force working between the first permanent magnet and the second permanent magnet, and wherein the first permanent magnet and the second permanent magnet are provided in opposed relationship with each other in a circumferential direction perpendicular to an axial direction of the bearing device, and one of the first permanent magnet and the second permanent magnet is of such a shape as to surround the other of the first and second permanent magnets.
Also, the deflecting-scanning apparatus according to the present invention is structured to include a deflector for deflectively scanning a light beam, and a rotating device for rotatively driving the deflector. The bearing device of the rotating device is structured to include a stator portion which includes one of a shaft and a sleeve, a rotor portion which includes the other of the shaft and the sleeve which are structured to be capable of relative rotation therebetween, a first permanent magnet mounted on the rotor portion, and a second permanent magnet mounted at a position opposed to the first permanent magnet, and wherein the rotor portion is floated up relative to the stator portion by a magnetic force working between the first permanent magnet and the second permanent magnet, and wherein the first permanent magnet and the second permanent magnet are provided in opposed relationship with each other in a circumferential direction perpendicular to an axial direction of the bearing device, and one of the first permanent magnet and the second permanent magnet is of such a shape as to surround the other of the first and second permanent magnets.
Also, in the above-described example of the prior art, as previously described, a dynamic fluid bearing is used in the bearing portion of the rotatable polygon mirror to thereby cope with the higher speed or the like of the apparatus, but there is a problem still left to be solved that if the pressure force of the leaf spring 33 for assembling the rotatable polygon mirror 34 to the flange 25 is strong, the bearing surface of the rotatable sleeve 24 will be deformed by the reaction force thereof and as the result, the dimension of the bearing gap will change and the performance of the dynamic pressure fluid bearing will be spoiled.
The present invention has been made in view of the above-noted unsolved problem peculiar to the prior art, and an object thereof is to provide a bearing device which can prevent the bearing surface of a rotatable sleeve from being deformed by the pressure force of coupling means such as a spring for assembling a rotatable polygon mirror to the rotatable sleeve to thereby spoil the bearing characteristic and can greatly contribute to improvements in the higher speed and rotational performance of the rotatable polygon mirror, and a deflecting-scanning apparatus using the same.
To achieve the above object, the bearing device of the present invention has a fixed shaft and a rotatable sleeve rotatably fitted to the fixed shaft, and is characterized in that the rotatable sleeve is provided with a level difference portion for locally changing the inner diameter of the rotatable sleeve.
Also, to achieve the above object, the deflecting-scanning apparatus of the present invention has a rotatable polygon mirror which is a deflector for reflecting a light beam, driving means for rotating it, dynamic pressure fluid bearing means provided with a rotatable sleeve rotatably fitted to a fixed shaft, and coupling means for coupling the rotatable polygon mirror integrally to the rotatable sleeve, and is characterized in that the rotatable sleeve is provided with an engagement portion for engaging the coupling means, and a level difference portion for locally changing the inner diameter of the rotatable sleeve in the engagement portion.
Also, design may preferably be made such that the inner diameter of the engagement portion of the rotatable sleeve is locally enlarged by the level difference portion.
Also, a magnet constituting a thrust bearing may preferably be assembled to the level difference.